There is an ongoing need for component miniaturization in radio communication devices. For example, smaller and more efficient components are needed for light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, pager devices and other devices for promoting rapid, efficient and flexible voice and data communication.
Filters are needed for a variety of such communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, computer and ancillary equipment linkages as well as a host of other, increasingly complex personal or equipment information sharing requirements. The desire to render increasingly complicated communications nodes portable, places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.
Surface acoustic wave (SAW) ladder filters are a popular choice for radios because of their low loss and small size advantages. Filter performance, however, is limited by SAW resonator quality factor, Q, and capacitance ratio, r. For example, given a constant ratio of Q/r, the tradeoff between insertion loss, bandwidth, and out-of-band rejection is defined. If the rejection is increased the insertion loss must increase and the bandwidth must decrease. If, however, the rejection requirements for the SAW filter are relaxed, better passband characteristics can be achieved.
It is very common for radio designers to require several rejection specifications (shown as 10,12,14 in FIG. 1) and an insertion loss bandwidth specification (shown as 16 in FIG. 1). Typically, one small band of frequencies far from the passband must be highly attenuated in the filter (shown as 14 in FIG. 1). For a SAW ladder structure to achieve such high rejection, the filter must be designed to reject all out-of-band frequencies at that high attenuation level. This restriction limits the insertion loss and bandwidth that can be achieved in the passband as discussed above. An improvement can be obtained by cascading a notch filter with the SAW ladder filter. In this way, the ladder filter rejection requirements can be relaxed.
FIG. 2 shows a prior art notch filter 22. A SAW ladder filter, as is known in the art, is typically coupled with the notch filter 22 which consists of a parallel connected capacitor 24 and a delay line 26. Previously, such a notch filter 22 has been realized entirely in a ceramic substrate. However, the ceramic notch filter configuration only provides a zero near the high attenuation band, but no poles in the passband. As a result, the prior art notch filter 22 degrades insertion loss performance in the operating passband of the radio.
What is needed is a notch filter configuration that, while providing a stopband at a particular frequency, does not degrade the passband response of an associated SAW ladder filter at a desired frequency. It is also desirable to provide a notch filter that can be implemented with fewer components and in a compact form.